The invention relates in general to an apparatus and method for determining the volatility of a fuel.
It is known in the art relating to automotive engines that a key gasoline characteristic for good driveability during the cold start period of engine operation is volatility. Volatility is especially important at the time an engine is started because the oxygen sensor is too cold to allow closed-loop control of the air-to-fuel ratio, the catalytic converter is too cold to efficiently oxidize hydrocarbon emissions in the exhaust, and because the intake manifold is too cold to rapidly evaporate all of the fuel that is injected. If too little gasoline is injected relative to the air intake, the engine has poor driveability; if too much gasoline is injected relative to the air intake, then extra hydrocarbons from an unburned portion of gasoline are found in the exhaust. Because gasoline sold in the United States varies in volatility, there is a tradeoff in engine design between low hydrocarbon emissions and good driveability with low volatility fuel.
To describe the effect of gasoline volatility on the cold start and warm-up driveability of a vehicle, a driveability index (xe2x80x9cDIxe2x80x9d) has been developed. Fuel with low DI is more volatile than fuel with high DI. In the United States, fuel is sold with DI that ranges from 910 to 1320. After being dispensed into a vehicle, fuel weathers as the more volatile constituents preferentially evaporate. This causes its DI to increase. Vehicle manufacturers take this wide variation in fuel DI into account. Engines are designed to meet requirements for low total emissions of hydrocarbons in the exhaust during the federal test procedure (xe2x80x9cFTP testxe2x80x9d), performed with tightly controlled calibration fuel, but engines should also provide satisfactory performance with the fuels that are actually used. Accurate control of the air-to-fuel ratio during the cold start period of engine operation helps achieve both of these goals. During the cold start period the air-to-fuel ratio is set in open loop control. Unfortunately, variation in the DI of fuel used in the United States limits the accuracy of open loop control of the air-to-fuel ratio during the cold start period since the intake manifold has not yet warmed up enough to evaporate all of the fuel that is injected.
Vehicle manufacturers presently address this problem in two ways. The first is to calibrate the engine fueling algorithm to provide extra fuel, so that acceptable cold start performance is experienced even with fuel that has volatility near the low end of the range encountered in the real world. One drawback of this approach is that it increases the vehicle""s exhaust hydrocarbon emissions on the FTP test. A large fraction of the remaining exhaust hydrocarbon emissions occur during the cold start period of engine operation. There is need for a cost effective way to decrease these emissions and meet the more stringent emission regulations coming into effect.
The second approach is to provide two calibrations for the engine. The default calibration is intended for use with the certification fuel used on the FTP test. A secondary calibration is provided that adds extra fuel to provide good engine performance with low volatility fuel, but causes higher exhaust hydrocarbon emissions. The engine is monitored during the cold start period for symptoms that are indicative of operation with high DI fuel. If such symptoms are detected, the engine switches from the default calibration to the secondary calibration. Thus, for the FTP test with certification fuel, the default calibration is used to obtain acceptably low exhaust hydrocarbon emissions. For real world operation with low volatility fuel, one symptom of a fuel-related problem triggers the use of the secondary fueling algorithm. Engine performance is adequate after the switch, but exhaust hydrocarbon emissions are increased.
If the volatility of the fuel (DI) were known from an on-board sensor then it would be possible to use the information to improve open-loop control of the air-to-fuel ratio. This would decrease exhaust hydrocarbon emissions, improve engine performance during the cold start period, decrease the delays associated with engine development, and improve fuel economy.
The present invention provides a method and apparatus for determining the volatility, specifically the driveability index, of a fuel to control engine operation based on measurements of capacitance obtained while a volume of fuel is heated. The method for determining the volatility of a fuel sample comprises the steps of collecting the fuel sample in a container proximate to a heater device; heating the fuel sample over a time period using the heater device; measuring a capacitance of the fuel sample periodically during the step of heating the fuel sample; determining a temperature of the fuel sample periodically during the step of heating the fuel sample; and determining the volatility of the fuel sample using the capacitance and the temperature.
In one aspect, the method further comprises the step of shutting off a fuel pump of a vehicle prior to the step of heating the fuel sample. This aspect can further include the step of storing a value for the volatility of the fuel sample until an ignition of the vehicle is turned on.
In another aspect of the invention, the step of collecting the fuel sample in a container comprises the step of collecting the fuel sample in a cup mounted on the heater device when a fuel pump is running. The step of heating the fuel sample over a time period using the heater device, in another aspect of the invention, can include the step of heating the fuel sample until a temperature of the fuel sample reaches a starting temperature plus a temperature change.
A preferred aspect of the invention exists where the step of heating the fuel sample over a time period using a heater device comprises the step of applying current to a heater of the heater device. The heater device further includes a dielectric body with a surface on which the container is mounted; a guard electrode on the surface electrically connected to the container; and two electrodes within the dielectric body operatively positioned to measure the capacitance of the fuel sample in the container. The heater is disposed within the dielectric body below the two electrodes. In this aspect, the heater is preferably a resistive heater with a known relationship of a resistance of the resistive heater to a heater temperature. In this aspect, the method can further include the step of operatively coupling a circuit to the two electrodes for measuring the capacitance of the fuel sample.
In another aspect of the invention, the step of measuring a capacitance of a fuel sample comprises the steps of operatively positioning two electrodes to measure the capacitance of the fuel sample in the container and operatively coupling a circuit to the two electrodes for-measuring the capacitance.
In yet another aspect of the invention, the step of determining a temperature of the fuel sample comprises the steps of operatively coupling a circuit to the heater device for measuring a voltage drop across the heater device; determining a resistance of the heater device using the voltage drop and a current applied to the heater device; and determining a heater temperature of the heater device based on a known relationship between the resistance of the heater device and the heater temperature; and wherein the heater temperature is the temperature of the fuel sample.
In the method of the present invention, the step of determining the volatility of the fuel sample using the capacitance and the temperature can comprise the step of comparing the capacitance and the temperature to experimental values for fuels with a variety of volatilities.
The method can optionally include the step of using a first measurement of capacitance to detect a concentration of oxygenate in the fuel sample. In a preferred aspect of the invention, the step of determining the volatility of the fuel sample comprises the step of determining a driveability index of the fuel sample.
The apparatus for determining a volatility of a fuel sample comprises a container for collecting the fuel sample proximate to a heater device, the heater device heating the fuel sample over a time period; means for measuring a capacitance of the fuel sample periodically during the step of heating the fuel sample; means for determining a temperature of the fuel sample periodically during the step of heating the fuel sample; and means for determining the volatility of the fuel sample using the capacitance and the temperature. Preferably, the container is a cup mounted on the heater device collecting the fuel sample when a fuel pump is running.
In one aspect of the invention, the heater device comprises a dielectric body with a surface on which the container is mounted; a guard electrode on the surface electrically connected to the container; two electrodes within the dielectric body operatively positioned to measure the capacitance of the fuel sample in the container; and a heater disposed within the dielectric body below the two electrodes. The heater is preferably a resistive heater with a known relationship of a resistance of the resistive heater to a heater temperature. In yet another aspect, this apparatus further includes a circuit operatively coupled to the two electrodes for measuring the capacitance of the fuel sample.
In another aspect of the invention, the means for measuring a capacitance of a fuel sample comprises two electrodes operatively positioned to measure the capacitance of the fuel sample in the container and a circuit operatively coupled to the two electrodes for measuring the capacitance.
In yet another aspect of the invention, the means for determining a temperature of the fuel sample comprises a circuit operatively coupled to the heater device for measuring a voltage drop across the heater device; means for determining a resistance of the heater device using the voltage drop and a current applied to the heater device; and means for determining a heater temperature of the heater device based on a known relationship between the resistance of the heater device and the heater temperature. The heater temperature is the temperature of the fuel sample.
In another aspect of the apparatus of the invention, the means for determining the volatility of the fuel sample using the capacitance and the temperature comprises means for comparing the capacitance and the temperature to experimental values for fuels with a variety of volatilities.